An analysis of metal wear in rock excavation by TBM

Abrasivity, or its reciprocal concept, "wear resistance", are not intrinsic properties of a material, but rather parameters describing the interaction of two materials, i.e. the wear part and the material exerting the wearing action. Moreover, "wear" or, at least, that part of the wear, which is due to the cumulated effect of minute scratches, is linked to the "hardness dispar-ity" concept: an example is provided by the well-known Mohs hardness scale (the "harder" body wears the "softer" body). In the case of rock-metal interaction, a difficulty arises from the inhomogeneity of the interacting bodies. The paper, after a synthetic explanation of the basic principles, deals first with the problem of measuring and representing the hardness of inhomo-geneous bodies; a simple procedure is described, with practical examples. Then, after a review of data on TBM disc service life and rock abrasivity evaluation, cases of TBM used in hydropower tunnel driving in Italy are presented. Data pertaining to the rock bored and to the metal composing the discs, together with machine data, tools consumption and machine productivity are provided and compared to find correlations enabling to forecast tools service life.